The invention relates to a solar cell, especially a multi-junction solar cell with an amount of pn-junctions stacked on top of each other.
Solar cells are large area pn-junctions. In so-called multi-junction solar cell designs, several pn-junctions are stacked on top of each other such that the band gap of the semiconductor materials constituting the junctions decreases from the top to the bottom of the solar cell. Typically one to three pn-junctions are stacked on top of each other.
FIG. 1 shows a cross section through a state of the art triple junction (TJ) solar cell 14. A Germanium (Ge)-wafer 4, typically having a thickness of 100 to 200 μm, serves as the substrate of the cell. A pn-junction 3 is created, some micrometers from the wafer front surface by diffusion. On top of the wafer, two more pn-junctions 1, 2 are grown from III-V semiconductor materials. In this configuration the three pn-junctions are located within a depth of approximately 10 μm from the cell surface (first main surface 31 of the semiconductor body). With 32 a second main surface of the semiconductor body is indicated. At a cell edge 33, the pn-junctions are exposed to the environment (cf. reference numeral 5 indicating a side wall of a trench as a result of manufacturing process). Although the above mentioned dimensions are approximately four orders of magnitude smaller than the lateral extension of the solar cell (typically in the range of some centimeters), these open pn-junctions in the area outlined with reference numeral 5 are critical.
Any damage occurring across pn-junctions will have an adverse electrical effect. It is irrelevant whether the damage is mechanically, chemically by species of the environment attacking the cell edge or physically by diffusion of foreign atoms into the junction or by the creation of a conductive path along the cell edge 33. This damage created across the pn-junction 1, 2, 3 acts electrically like a local shunt and impacts the fill factor of the solar cell 14. The fill factor is a measure of how rectangular the so-called IV-curve of the cell is. In comparison, the same damage process occurring on the cell surface, e.g. on the top most n doped material of pn-junction 1 in FIG. 1, has a much smaller impact.
In addition, there are straightforward ways to passivate the top surface of the solar cell, e.g. by growth of appropriate semiconductor materials or oxides. However, such a passivation layer 6 is not possible at the cell edge 33. A contact grid 7 made from metal is located on the passivation layer 6, sometimes with an additional semiconductor layer underneath (cap layer). Sometimes the contact grid 7 is referred to as frontside grid. A contact pad 8 is created as part of the contact grid 7. Furthermore, an antireflection coating 9 is applied on top of the passivation layer 6 and the contact grid 7, respectively.
To separate the solar cell from unwanted areas of the wafer 4, the wafer is mechanically cut. In order to prevent mechanical damage at the edge of the pn-junctions with the detrimental effects outlined above, a trench is etched into the wafer 4 to a certain depth around the circumference of the solar cell. This trench is typically termed “mesa groove”. It has a depth of approximately 10 μm in the case of the solar cell of FIG. 1. Together with the mechanical cut represented by the edge 10 of the wafer 4, a small step 11 at the edge of the cell is created. As shown in FIGS. 3, 5 and 6, a depth of the mesa groove 19 may be greater than a depth of step 11 which is disposed adjacent to the sidewall 5 of the mesa wall 20. The open pn-junctions ending at the sidewall 5 of the trench are free from sawing damage. This process is state of the art in III-V solar cell technology.
Individual solar cells as described above are interconnected in series and in parallel to form a solar cell module which provides the desired voltage and current for a particular application. A series interconnection of three solar cells with individually encapsulated cells is shown in FIG. 2. The principle illustrated in the sectional view is typical for solar arrays for space use. Since the structure of the three solar cells is identical, the following description only refers to the solar cell 14 shown in the middle.
As environmental protection, the solar cells are usually enclosed in a transparent encapsulant and equipped with a glass sheet as common front surface. There are cases, where a common encapsulation of all solar cells is not possible, most notably if the solar cells are exposed to large temperature fluctuations while bonded 12 to a substrate 13 with a large thermal expansion mismatch. Solar cells on a solar array for space are an example for this situation. Therefore, each solar cell 14 is equipped with its own coverglass 15 with the help of a transparent, non-conductive adhesive 16. While the coverglass 15 typically is made from a 100 μm thick Cerium doped microsheet, the non-conductive adhesive 16 has a thickness of around 20 μm. For space arrays typically silicones are used as transparent encapsulants, for example DC93-500 from Dow Corning, RTV-S690 from Wacker or CV16-2500 from Nusil. Individual cells 14 are connected in series by a cell interconnector 17, for instance a metal stripe. The cell interconnector 17 connects a contact grid and a contact pad 8, respectively, of one solar cell 14 with a backside metallization 18 of the neighbouring solar cell. In this configuration, the pn-junctions ending at the side wall 5 of a mesa groove at the cell edge 33 remain exposed to the environment. The non-conductive adhesive 16 is typically not intended to protrude further into the inter cell gap to avoid adverse effects on the fatigue behaviour of the cell to cell interconnectors 17.
U.S. Patent Application Publication No. 2009/0320917 A1, however, discloses the possibility to passivate and protect the cell edge by the encapsulant itself.
Exemplary embodiments of the present invention provide a solar cell, especially a multi-junction solar cell, for space applications, comprising an improved edge protection.
In accordance with the present invention, a solar cell, particularly a multi-junction solar cell, comprises a first main surface and a second main surface opposite to the first main surface of a semiconductor body wherein a topmost pn-junction of an amount of pn-junctions stacked on top of each other adjoins to the first main surface. The solar cell may comprise only one pn-junction or a plurality of pn-junctions. Typically, a solar cell comprises between one and three pn-junctions. The solar cell according to the invention can be, in particular, a triple junction solar cell for space applications.
The solar cell further comprises a cell edge of the semiconductor body defining a shape of the first and the second main surfaces and thus the solar cell. The cell edge of the semiconductor body may be stepped or plane.
The solar cell further comprises an encapsulant provided on the first main surface for providing an environmental protection of the semiconductor body. The encapsulant may be a transparent, non-conductive adhesive.
Furthermore, a mesa groove is provided on the first main surface and penetrating at least the topmost pn-junction, wherein the mesa groove is located adjacent to the cell edge and created around the circumference of the semiconductor body for providing an inner cell area and a mesa wall. The mesa wall is created between the mesa groove and the cell edge. The mesa groove is filled with the encapsulant.
The mesa wall serves as an edge protection and as an environmental protection for the inner cell area. Due to the mesa groove being located adjacent to the cell edge and created around the circumference of the semiconductor body the active cell edge is not longer exposed to the environment and protected from mechanical, chemical or physical attacks.
The mesa groove according to the invention may therefore be called an inner mesa groove which separates the semiconductor body in an inner cell area and a mesa wall adjoining to the cell edge with its wall being on the opposite side of the inner cell area. This area of the mesa wall is part of the cell edge which might be stepped due to an outer trench etched into the wafer primary before the wafer is mechanically cut into single solar cells. However, this area of the mesa wall which still is exposed to the environment is not an active part of the solar cell. Manufacturing of a solar cell may be made with known technologies without the need to change manufacturing steps.
According to an exemplary embodiment, the inner cell area is completely, i.e. continuously, surrounded by the (inner) mesa groove.
In a further exemplary embodiment all pn-junctions of the amount of pn-junctions are penetrated by the (inner) mesa groove so that the mesa wall is electrically isolated from the inner cell area. This has the advantage that the active, inner cell area is electrically isolated by the mesa groove and the encapsulant from the cell edge.
According to a further exemplary embodiment the mesa wall is at each depth of the mesa groove mechanically attached to the inner cell area. With this embodiment the maximum depth of the mesa groove is specified. In a triple junction solar cell having three pn-junctions a depth of 10 to 20 μm is sufficient because a bulk of the wafer of the solar cell adjoining to the second main surface is not electrically active. The width of the mesa wall is determined by a trade off between manufacturability and loss in active cell area. It will be understood that the mesa wall has to have sufficient strength to be self-supporting.
According to a further exemplary embodiment the width of the mesa groove is chosen dependent on the viscosity of the material of the encapsulant. In one design, where the encapsulant is meant to fill the mesa groove completely, the minimum width of the mesa groove depends on the viscosity of the encapsulant. In an alternative embodiment the maximum width of the groove is chosen so small that the encapsulant does not fill the trench before it is cured. Both alternative design variations provide a complete encapsulation of the pn-junctions adjoining the (inner) mesa groove from environmental attacks.
According to a further embodiment, the width of the mesa wall is adjusted in its dimensions for blocking UV (ultraviolet) radiation from side if the solar cell is operated under inclined conditions.
According to a further exemplary embodiment the material of the encapsulant is non-conductive.
According to a further exemplary embodiment a contact grid is provided on the first main surface of the semiconductor body only within the inner cell area. It is to be noted that the contact grid is not provided on top of the mesa wall.
Furthermore, a non-conductive antireflection coating may be provided on the inner cell area covering the contact grid and on the top of the mesa wall.
According to a further preferred embodiment a coverglass is located on the encapsulant for sealing the solar cell and the mesa groove on the first main surface. It is apparent to a person skilled in the art that the coverglass will be located on the encapsulant before a plurality of solar cells has been bonded to a substrate.
Preferably, the coverglass comprises an UV blocking filter for preventing photodegradation of the encapsulant in the groove.
According to a further embodiment the cell edge exposed to environment is coated with a non-UV transparent material around the entire cell circumference.
According to a further embodiment a conductive or antistatic material for providing an electrical shunt path is applied on the cell edge which extends between a back side of the solar cell and a front side of a solar cell, the back side being provided by the second main surface and the front side being provided by the coverglass, thereby at least contacting the encapsulant and, the mesa wall. The conductive or antistatic material may be a coating, a paint or a film. The aim of the conductive or antistatic material is to reduce or prevent the build-up of electric charge on the coverglass and thus to guard against electrostatic discharge (ESD). The conductive or antistatic material may be applied at only one position at the cell edge. For example, cropped corners are an ideal location since the distance to neighbouring solar cells on an array or solar module is the largest.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.